This invention relates to methods for extruding polymers and, more particularly, to extrusion methods which reduce the incidence and/or degree of melt fracture in linear polyethylene homopolymers and copolymers by variations in the geometry of the extrusion die exit face.
"Melt fracture" is a term used to describe the surface irregularity which occurs during extrusion of some thermoplastic materials under certain conditions of flow rate and melt temperature. It is characterized by a series of ridges perpendicular to the flow direction and is described, for example, by J. A. Brydson, Flow Properties of Polymer Melts, pages 78-81, Van Nostrand-Reinhold (1970). Melt fracture may be formed during extrusion of several different thermoplastic materials, notably, linear polyethylene homopolymers/copolymers. Within a film extruder and extrusion die, such resins undergo severe shearing deformation with the result that melt fracture can result. To lessen this possibility, the extruder can be operated at a reduced flow rate but this expedient carries with it the penalty of reduced productivity.
A number of approaches to reducing melt fracture and a particular kind of melt fracture referred to as "sharkskin" have been explored and are discussed, for example, in U.S. Pat. Nos. 3,879,507; 3,920,782; 4,187,269; 4,267,146; 4,282,177; 4,348,349; and, 4,360,494. Other publications exploring the phenomenon of melt fracture are Tordella, "Capillary Flow of Molten Polyethylene--A Photographic Study of Melt Fracture", Transaction of the Society of Rheology, Vol. 1, 203-212 (1957) and Vlachopoulos et al., "A Comparison of Melt Fracture Initiation Conditions in Capillaries and Slits", Journal of Applied Polymer Science, Vol. 21, 1177-1187 (1977).